The radiography detectors are usually powder phosphor screens or needle image plates (needle IP), direct radiography detectors fabricated from, for example, amorphous silicon, amorphous selenium, complementary metal oxide semiconductor (CMSO), phosphor detector arranged for direct radiography, or the like.
A radiation image is recorded on such a radiography detector (also called ‘plate’) by exposing it to an X-ray field. The radiation image which is temporarily stored by the detector is read out in a so-called read out system (also called ‘digitizer’) where the exposed detector is scanned with light of an appropriate wavelength and where the image-wise modulated light emitted by the detector upon stimulation is detected and converted into a digital image signal representative of the radiation image.
A phosphor screen or needle image plate is commonly conveyed in a cassette and is not a part of the read out system. This implies that for each detector the uniformity of the detector needs to be evaluated.
One aspect of this uniformity is the sensitivity of the detector in each point of the detector surface relative to the average sensitivity. This notion is referred to as intrinsic uniformity. This is expressed as a percentage of the average signal emitted by the screen for a constant dose in each point of the detector surface.
Each detector can also be evaluated to determine whether the sensitivity is within specifications, i.e. whether for a given dose the signal that is emitted by the detector is within specifications.
The American Association of Physicists in Medicine Task Group #10 on Computed Radiography (abbreviated AAPM) discusses receptor reproducibility, density uniformity and artifact analysis. It is declared by this organization that the intrinsic and receptor to receptor uniformity are expected to be homogeneous and consistent. The group sets out a procedure for detector testing and also defines acceptance levels.
The prescribed procedure relates to exposure of all detectors part of a detector inventory whereby all detectors are centered relative to the axis of an incident x-ray beam. The detectors are uniformly irradiated over the entire plate surface. The signal read out of the detectors by a calibrated read out system is then evaluated relative to a pre-defined acceptance level.
IEC published a test procedure to evaluate inter-plate sensitivity variations. The test is to confirm that inter-plate sensitivity variations of storage phosphor system are within a specified range and meet the manufacturer's specifications, or the regulatory or contractual requirements.
The AAPM group has specified that with regard to non-uniformity a 10% peak-to-peak variation is acceptable and also for specifications of sensitivity a 10% variation is acceptable.